Stalin's autopsy showed that he had died of a simultaneous stroke in the brain and a stomach haemorrhage, the co-happenstance of which seems unlikely - that is, unless Stalin had taken a high dose of warfarin. Beria openly declared he had killed Stalin in an effort to shore up support for him to be his successor. However, even under interrogation (and presumably torture) he never made mention of poison of any kind.
After doing some rudimentary reading, it looks like there are conflicting views on this (with what seems to be Sebag-Montefiore vs. everyone else).
Was it ever proved that Beria killed Stalin, either by his own hand or orders given to the NKVD?
If there was any hard evidence there would be no bickering between the natural death- and the conspiracy-theorists. The single most important point to the conspiracy theorists is the indictment of his former head of personal security (who previously served in this position for 25 years) three months before Stalin has died was killed. Problem is it happened with Stalin's approval. So the conspiracy theory states he was tricked into replacing his head of palace guard by [insert here the supposed mastermind of the conspiracy].